Michael Staines, a physicist at Industrial Research Ltd, Lower Hutt,
who has worked with sea ice in the Antarctic, responded.

Whether a material is transparent or coloured depends on the types of atoms in
the material and how they bind together.

Light is a type of wave which exerts a force on an electrically charged object,
such as an electron in an atom. Like a wave on the surface of the sea, it
carries energy. In the case of light waves the amount of energy depends on the
frequency - that's the number of wave crests passing per second if you could
count them as the wave goes by. Red light, for example, has a lower frequency
and lower energy than blue light.

We and all the things around us are composed of atoms which contain electrons.
As it travels past, a light wave will exert forces on these charged particles,
pushing them to and fro at the frequency of the wave.

So why don't the charged particles in matter absorb all the energy from any
passing light waves? It turns out, that in much the same way as a playground
swing will only get going if you give it a little push each time it comes back
to you, atoms and molecules can only absorb energy at particular frequencies.

In the case of water, none of these absorption frequencies are in the range of
visible light so water is clear. But if our eyes could see light of lower
frequency, in the infrared, or higher frequency, in the ultraviolet, water
would appear brilliantly coloured.